Apparatus and method for biasing cascode devices in a differential pair using the input, output, or other nodes in the circuit

ABSTRACT

A differential amplifier is configured in a cascode configuration that includes input transistors that are connected to corresponding cascode transistors. The gates of the cascode transistors are tied together to form a common bias for the cascode devices. The input transistors of the differential amplifier receive a differential input signal that is amplified and outputted to an output circuit that is connected to the cascode transistors. The cascode devices require a bias voltage for proper operation. Preferably, the bias voltage puts the cascode devices into the saturation region. The gates of cascode devices are coupled together and connected to a bias terminal. In embodiments of the invention, the bias terminal is connected to another terminal of the chip to provide the bias for the cascode devices.

This Application claims the benefit of U.S. Provisional Application No.60/639,304, filed on Dec. 28, 2004, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to cascode devices, and morespecifically to biasing cascode devices without an external biasingcircuit.

2. Background Art

Communications circuits often include a differential configuration anddifferential amplifier to improve noise performance. Using a cascodeconfiguration in a differential pair is a common practice. Theadvantages include, but not limited to, providing more isolation fromthe input to the output of the trans-conductor stage, higher impedanceat the output of the input trans-conductor thereby boosting the gain,and protecting the drain of the input devices. As a result, the cascodeconfiguration is quite popular in differential pairs. However, thecascode devices require a bias voltage. This is usually done by addingsome circuitry particularly for this purpose which is undesirable forvery low-power applications.

Therefore, what is needed is a cascode configuration in a differentialamplifier with a low power bias circuit configuration.

BRIEF SUMMARY OF THE INVENTION

A differential amplifier is configured in a cascode configuration thatincludes input transistors that are connected to corresponding cascodetransistors. Specifically, the drains of the input transistors areconnected to corresponding sources of the cascode transistors. The gatesof the cascode transistors are tied together to form a common bias forthe cascode devices, and the drains are coupled to an output circuit.The differential amplifier receives a differential input signal foramplification and produces an amplified signal that coupled to theoutput circuit. The cascode devices provide more isolation from input tooutput, and a higher impedance at the output of the input devices,thereby boosting gain.

The cascode devices require a bias voltage for proper operation.Preferably, the bias voltage puts the cascode devices into thesaturation region. The gates of cascode devices are coupled together andconnected to a bias terminal. In embodiments of the invention, the biasterminal is connected to another terminal of the chip to provide thebias for the cascode devices. This can include the input and outputnodes if they have a well-defined and relatively fixed voltage. In oneembodiment, the output circuit includes a second stage amplifier thatgenerates the DC voltage necessary to bias the cascode devices. In thisway, there is no need for external bias circuitry.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 illustrates a differential amplifier in a cascode configurationwith a common bias terminal for the cascode devices according toembodiments of the present invention.

FIG. 2 illustrates a differential amplifier in a cascode configurationwhere the cascode devices are biased using the second stage amplifieraccording to embodiments of the present invention.

FIG. 3 illustrates a differential amplifier in a cascode configurationwhere the cascode devices are biased using the output terminal of theoutput circuit according to embodiments of the present invention.

FIG. 4 illustrates a differential amplifier in a cascode configurationwhere the cascode devices are biased using the input terminals.

FIG. 5 illustrates a bandgap circuit that includes a differentialamplifier with self-biased cascode devices according to embodiments ofthe present invention.

FIG. 6 further illustrates bandgap circuit of FIG. 5 according toembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a differential configuration 100 having a inputdifferential amplifier 106 and an output circuit 102 that areimplemented on a common substrate, for example a CMOS substrateaccording to embodiments of the present invention. The differentialamplifier 106 and the output circuit 102 could also be referred torespectively as an input amplifier stage and an output amplifier stage,or first and second stages or circuits, for convenience.

The differential amplifier 106 is configured in a cascode configuration.Namely, the differential amplifier 106 includes input field effecttransistors (FET) M1 and M2 that are connected to corresponding cascodedevices M3 and M4. M3 and M4 are also FET devices that are cascodeconnected to the corresponding FET devices M1 and M2.

Specifically, the drains of M1 and M2 are connected to the correspondingsources of M3 and M4. The gates of M3 and M4 are tied together andbiased with bias node 107, such that the bias node 107 provides a gatebias for the cascode FET devices M3 and M4.

The input devices M1 and M2 receive a differential input signal atterminals 108 a and 108 b that is amplified and outputted to the outputcircuit 102. As discussed above, the cascode devices M3 and M4 providemore isolation from input to output, and a higher impedance at theoutput of the input devices M1 and M2, thereby boosting gain. The outputcircuit 102 includes transistors M5 and M6 that are connected to thecascode devices M3 and M4, where the output of the first stage ofamplification is taken from the drain of M6. The drain of M6 is coupledto a second stage of amplification 104, the output of which is connectedto the output terminal 105.

As discussed above, the cascode devices M3 and M4 require a bias voltagefor proper operation. Preferably, the bias voltage puts M3 and M4 intothe saturation region. The gates of cascode devices M3 and M4 arecoupled together and connected to the bias terminal 107. In embodimentsof the invention, the bias terminal 107 is connected to another terminalof the chip to provide the bias for the cascode devices M3 and M4. Thiscan include the input and output nodes if they have a well-defined andrelatively fixed voltage, or any other node in the circuit 100 if they awell-defined and relatively fixed voltage. Further, the two stageamplifier can be an operational amplifier that provides a well definedand relatively fixed output voltage.

For example, FIG. 2 illustrates that the second stage amplifier 104generates a DC voltage that is connected to the bias terminal 107 tobias the cascode devices M3 and M4. In other words, the second stageamplifier 104 generates a fixed voltage at node 109 that is sufficientto bias the cascode devices M3 and M4 into the saturation region. FIG. 3illustrates that the output terminal 105 generates a fixed DC voltagethat is sufficient to bias the cascode devices M3 and M4 into thesaturation region. It is noted that M3 and M4 are FET devices that drawno current at their respective gates.

Still referring to FIGS. 2 and 3, connecting the output 105 to the inputof the differential amplifier 106 would normally cause stabilityconcerns. However, stability concerns are mitigated since the gates ofM3 and M4 are tied together so that only a common mode voltage is fedback.

In other words, M3 and M4 are configured for differential amplification.

However, no differential signal is fed-back for amplification.Furthermore, the impedance seen by the sources of M3 and M4 isrelatively large due to the respective drain impedances of M1 and M2.Therefore, the voltage at the bias terminal 107 is primarily droppedacross the source impedance seen at the sources of M3 and M4, causingthe gain from gate-to-drain of M3 and M4 to be minimal.

FIG. 4 illustrates that the input terminals 108 a and 108 b generatesufficient voltage to bias the transistors M3 and M4, provided that M3and M4 are low threshold devices. In other words, the input signals atthe input terminals 108 a and 108 b have a DC component sufficient tobias low threshold devices. In another embodiment, the DC component ofthe input signals is higher so that conventional FETs can be biased withthe input terminals 108 a and 108 b.

FIG. 5 illustrates an embodiment, where the circuit 100 is implementedin a voltage reference circuit 500 that generates a stable outputvoltage V_(ref) 504. V_(ref) 504 is stable and is sufficient to put thecascode devices M3 and M4 into saturation, and therefore can be used tobias the cascode devices M3 and M4 as discussed below.

Bandgap circuit 500 includes: resistors R₁, R₂, and R₃; diode connectedbi-polar transistors Q₁ and Q₂; and an amplifier 502. The bandgapcircuit 500 generally operates as follows. The diodes Q₁ and Q₂ aresized to conduct a relative current of 1:n, as shown in FIG. 5. The opamp 502 senses the voltages at nodes X and Y and drives R₁ and R₂ sothat V_(X)=V_(Y). Given these conditions and if R₁=R₂, it can be shownthat V_(BE1)−V_(BE2)=V_(T) ln n where V_(BE) is the base-emittervoltage. This forces a current through R₃ that is equal to V_(T) ln(n)/R₃, where V_(T) is the threshold voltage of diodes Q₁ and Q₂. Thisresults in an output voltage of:V _(ref) =V _(BE2)+(V _(T) ·ln n) (1+R ₂ /R ₃).

As a result, V_(ref) 504 is relatively stable, and the voltage amplitudeis determined by the current and device ratio n, and by the ratio ofR₂/R₃. Bandgap circuits are further described in Design of Analog CMOSIntegrated Circuits, Behzad Razavi, McGraw Hill, 2001, which isincorporated herein by reference in its entirety.

Still referring to FIG. 5, the amplifier 502 is configured using thedifferential amplifier configuration 100 of FIG. 1, where the inputterminals 108 a and 108 b are connected to the nodes X and Y, and theV_(ref) output 504 is taken from the output 105 of the second stageamplifier 104.

Since V_(ref) 504 is sufficiently stable, it can be connected to thebias terminal 107 and used to bias the cascode devices M3 and M4.

Accordingly, there is no explicit or extra bias circuit that is neededto bias the gates of the cascode devices M3 and M4, since the bandgapoutput V_(ref) is actually used to the provide the gate bias. Thus,there is significant savings in die area and power by eliminating theextraneous bias circuit for the cascode devices.

FIG. 6 provides a more explicit illustration of the bandgap circuit 500having the differential amplifier 100 from FIG. 1. Again, as discussedabove, the V_(ref) 504 is a stable voltage reference that is used tobias the cascode devices M3 and M4 that are part of the inputdifferential amplifier 106, which obviates the need for a separatecascode bias circuit.

Referring to FIG. 2-6, any of the nodes or terminals in the outputcircuit or elsewhere can provide the DC voltage to bias the cascodedevices M3 and M4, as long as the bias voltage is sufficient to put thetransistors M3 and M4 into saturation. The choices shown in FIGS. 2-6are only examples. Those skilled in arts will recognize that other nodescould be used to provide the bias voltage and these other configurationsare also within the scope and spirit of the present invention.

The differential amplifier configurations in FIGS. 1-6 can beimplemented in any type of CMOS process or other processes. Thedifferential amplifiers can be implemented in bipolar processes if thebase current needs of the bipolar transistors are considered andaccommodated.

CONCLUSION

Example embodiments of the methods, systems, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such other embodiments will be apparent to personsskilled in the relevant art(s) based on the teachings contained herein.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A differential amplifier, comprising: an input circuit including, a differential pair of transistors having respective gates that receive an input signal; a cascode pair of transistors connected to respective drains of said differential pair of transistors, and having respective gates connected together; and an output circuit connected to respective drains of said cascode pair of transistors, said output circuit providing a DC bias voltage for said gates of said cascode pair of transistors, wherein said output circuit is a portion of a bandgap circuit that generates said DC bias voltage based on a base-emitter junction voltage and a resistor ratio of said bandgap circuit.
 2. The differential amplifier of claim 1, wherein an output terminal of said output circuit is connected to said gates of said cascode pair of transistors.
 3. The differential amplifier of claim 1, wherein said output circuit includes an amplifier that provides said DC bias voltage.
 4. The differential amplifier of claim 3, wherein an output terminal of said amplifier provides said DC bias voltage.
 5. The differential amplifier of claim 1, wherein said cascode pair of transistors have respective sources connected to respective drains of said differential pair of transistors.
 6. The differential amplifier of claim 1, wherein said cascode pair of transistors are field effect transistors. 